This invention relates to bourdon tube transducers.
There are very few transducers that provide information which may be utilized directly by the users without a basic transformation of the output. In most cases, the transformation consists of some form of amplification and indication for measuring the output of the transducer. The bourdon tube is a basic transducer which transduces either a pressure or temperature unit area into a mechanical angle displacement. Depending upon the type of bourdon tube used, especially the geometrical configuration, either mechanical or electromechanical devices are used to amplify the bourdon tube's output. Where the application of the bourdon tube calls for amplification, the tube size must be increased to provide additional force to drive the amplifying device. For example, where mechanical amplification is used and the angular displacement is increased in size, the bourdon tube is a helical or spiral tube of a number of turns sufficient enough to provide the desired angular displacement. In the cases where the bourdon tube is limited to a single turn or a fraction thereof, the diameter is enlarged to increase the output displacement. This is, of course, due to the fact that when the bourdon tube is coupled to a mechanical or an electromechanical amplifying system, it has to provide an increase in the output torque to drive the mechanical or electromechanical amplifying system. To increase the torque, the size of the oval tube which constitutes the bourdon tube, is enlarged. Whenever these solutions are chosen, the weight of the bourdon tube will inevitably increase. This, in turn, will introduce a weight effect deviation to the performance curve of the bourdon tube. Similarly, when helical or spiral configuration tubes are used, not only is there the weight effect on deviation, but the fact that in response to the spurious mechanical stimulations, the bourdon tubes will oscillate. The resonant frequency of oscillation is inversely proportional to the weight of the bourdon tube. In order to suppress these mechanical oscillations, mechanical or fluidic dampers are used. These will, of course, increase the weight and result in further degradation of the performance curve of the bourdon tube. The bourdon tube is a spring in nature and its output equation contains a coefficient of the modulous of elasticity. Depending on the composition of the alloy used to make the bourdon tube, the output performance curve varies with temperature and pressure.
In particular, in the production area of the petroleum industry, bourdon tubes are used to measure temperature and pressure within the oil well or "down hole". Due to the fact that the oil well requires an apparatus in cylindrical geometry with a diameter as small as possible, the physical shape of the bourdon tube is critical, especially if there are the additional torque requirements, in which case the bourdon tube becomes helical. In general, the size of the bourdon tube for down hole metering varies from 2/10th's of an inch OD, with three inches length, to 9/10th's of an inch OD with a 15 inch length. The helical configuration exhibits tendencies to oscillate. The additional weight, lack of geometrical symmetry and temperature and/or pressure deviations for the down hole instrument result in limited accuracy and/or range of operation. The majority of accuracy problems originate from the intolerance of the mechanical linkage of the bourdon tube to the meter indication and hysteresis.
Inherently, the bourdon tubes do not have any effective hysteresis. However, due to improper design and processing, they will exhibit hysteresis as high as a few percent of the overall accuracy. All of these factors tend to contribute to the overall accuracy and reliability of the bourdon tube as a transducer. However, it has been found that the bourdon tube is the preferred transducer to use for down hole metering when compared to other instruments.
The traditional output of many evironmental condition sensing apparatus is an analog device which limits the accuracy to a few percent of the full scale readings. There is, however, an instrument which digitizes the primary output of the bourdon tube with an optical encoder, reference may be made to U.S. Pat. No. 3,968,691. The calibration curves of this apparatus distinctly indicate the existence of the weight and temperature degradation of the bourdon tubes. Some manufacturers choose strain gage transducers in order to avoid vibratory and temperature degradation as well as size limitation without improving the accuracy as much as they would like to. The strain gage transducers also have a 4/10th's of a percent hysteresis error as well as other errors.